Durp is commonly used as a way to greet a companion, usually when entering a room. Durp is the natural progression from other such greetings (see below). Also it can be said in approval of an opinion or statement or when showing off about something.
What is up? -> What's up? -> Wassup? -> Waddup? -> Sup? -> Durp?

A word created by Matt Stone and Trey Parker, some time ago starting around the time of cannibal the musical. it is a term of saying something along the lines of "oh shit i am fucking retarded". More people might recognize it the word with the character Mr. Durp whom replaced Chef, and when he would say something stupid would hit himself in the head with a fucking hammer.